Increasing demands in fuel efficiency have made hybrid systems more attractive in the automotive industry. In addition to a conventional combustion engine, an electric motor is an important part of the hybrid system. Alternating current (AC) induction motors are commonly used because they offer simple, rugged construction, easy maintenance and cost-effectiveness. The AC induction motor includes two major assemblies: a stator and a rotor. The stator is the outermost component of the motor and is composed of steel laminations shaped to form poles, with copper wire coils wound around the poles. The primary windings are connected to a voltage source to produce a rotating magnetic field. The rotor (often referred to in one form as a squirrel cage rotor) is a cylinder that is mounted on a shaft or mandrel to electromagnetically cooperate with the stator. The rotor is formed of longitudinal conductor bars cast into generally peripheral slots and cast together at both ends with short rings forming a cage-like shape. FIG. 1 shows an illustration of a notional induction motor 1 with a cast rotor 3 that may spin in response to changes in a magnetic field in stator 5. The core of the rotor 3 is built with stacks of electrical grade steel laminations 4 and aluminum or copper alloy rotor bars 7 are cast through conducting bar slots formed in the laminations 4 and end rings 9 creating an integrated squirrel cage structure.
As depicted in FIG. 1, a rotating magnetic field around the rotor 3 is generated from the field windings 11 in the stator 5 of an induction motor 1. Electric current is generated in the conductor bars 7 from the relative motion between the rotor 3 and the rotating magnetic field around the rotor 3. These lengthwise-flowing electric currents in the conductor bars 7 react with the magnetic field of the motor 1, producing force acting at a tangent to the rotor 3. This results in torque to turn the shaft or mandrel 20 and the rotor 3. In operation, the rotor 3 is carried around with the magnetic field, but at a slightly slower rate of rotation. The difference between the speed of the rotor and the speed of the magnetic field is called slip, and the slip increases with load.
Conductor bars 7 are usually skewed slightly along the length of the rotor 3 (i.e., the conductor bars 7 are not perpendicular to the plane of the end rings 9 where the end ring attaches to the conducting bars 7), as shown in FIG. 1. This results in the reduction of noise and also smoother torque fluctuations. Torque fluctuations can result in some speed variations due to interactions with the pole pieces of the stator 5. The extent to which the induced currents are fed back to the stator field winding coils 11, and thus the current through the coils, is determined by the number of conductor bars 7 on the squirrel cage. Constructions that use a prime number of bars offer the least feedback.
The iron core (laminated steel stack) carries the magnetic field across the motor. The structure and materials for the laminated steel stack are specifically designed to minimize magnetic losses. The thin laminations (electrical steel sheets), separated by an insulating coating, reduce stray circulating currents that would result in eddy current loss. Further reducing eddy-current loss is the fact that the material for the laminations is a low carbon, high silicon steel with several times the electrical resistivity of pure iron. The low carbon content makes it a magnetically soft material with low hysteresis loss.
The same basic design is used for both single-phase and three-phase motors over a wide range of sizes. However, the depth and shape of bars for three-phase motors will have variations to suit the design classification.
A common aluminum squirrel cage induction rotor construction method with a conventional two-plate high pressure die casting tool starts with an iron core of stacked thin stamped coated steel laminations compressed to a specified height and clamp pressure. Importantly, the lamination stack must be held and accurately compressed. Without proper lamination stack height compensation an assembly of too many laminations could prevent full die closure resulting in a large casting flash. An assembly of too few laminations can result in low compression force on the lamination stack causing metal to penetrate between laminations and under the mandrel, potentially causing tooling damage. Furthermore, lamination stacks compressed below specified pressure allow for infiltration of molten aluminum between individual laminations resulting in increased eddy current losses thereby reducing motor efficiency. Lamination stacks compressed at too high of pressure can result in damage to lamination insulation, also resulting in increased eddy current loss thereby reducing motor efficiency. Additionally, lamination stacks compressed at too high of pressure can increase conducting bar tension stress resulting from lamination stack spring back causing rotor distortion and loss of durability during use.
Therefore, there is a need for an integrated compensation device assembly for lamination stack height for use in a conventional two-plate high pressure die cast tool used for casting aluminum induction rotors that will allow for variation in lamination stack height and ensure constant clamping pressure on the steel lamination stack, as well as for improved methods of compensating for lamination stack height variation in the manufacture of die cast aluminum induction rotors.